Although Simplenote now has an official OS X app [¹] I'd strongly suggest nvALT [²] which is essentially Notational Velocity with some nice additional features and under active development.
45 karma · joined November 20, 2013
[1] http://scr.im
[2] http://ozh.org
[1] https://encrypted.google.com/search?q=site%3Assl.bing.com